That was the title of an article written by H. Claire Brown in the New York Times a couple years ago. The sub-heading read “Herbicides are losing the war … and agriculture might never be the same again.”
Brown began by explaining the concept of superweeds: “Weeds that have evolved characteristics that make them more difficult to control, as a result of using the same management tactics, are rapidly taking over American commodity farms – and Palmer amaranth (Amaranthus palmeri) is their king.”
She wrote that scientists have identified populations of A. palmeri that can tolerate being sprayed with six different herbicides (though not all at once), and that they continue to develop new resistances.
Soon it became clear that weeds were evolving faster than chemical companies were able to develop new weed killers. Just eight years ago, in response to the onset of resistance to glyphosate (the ag chemical industry’s most commonly used herbicide active ingredient), its St. Louis-based corporate owners changed their game plan. They began selling a new generation of genetically modified seeds, bred to resist both glyphosate and dicamba. Dicamba, a 60-year-old herbicide at the time, was resurrected to substitute for the increasingly useless glyphosate.
Nonetheless, by 2020, scientists had confirmed the existence of dicamba-resistant A. palmeri. The agribusiness giant had taken a decade to develop new seed product lines for several different field crops, but this weed had successfully mutated to become dicamba-immune in just five years.
Research published by the Weed Science Society of America found that uncontrolled weeds could cause tens of billions of dollars of crop loss every year. Weed scientists at Iowa State estimated that the tipping point, at which weed killers cease to be effective on some particularly pernicious species (including A. palmeri) is less than 10 years away. Quoting these workers: “There’s general consensus among weed scientists that the problems we see are just going to continue to accelerate. That’s why we question whether we can continue this herbicide-only system.”
To put more of a face on this problem, Vipan Kumar, a weed scientist from Kansas State, gave Brown a tour of his research lab. He showed her an arm-length seed pod on a top-heavy A. palmeri plant, which Brown described as being “one of many, among rows and rows of tall, thin seedlings that shot up from small plastic trays, designed to support just a few inches of growth, arcing past light fixtures, as they stretched toward the window-paneled ceiling.”
The subject plant in his hands was an A. palmeri descendent that had demonstrated resistance to 2,4-D, one of two active ingredients in Agent Orange, the herbicide blend used to defoliate forests and jungles during the Vietnam War. The botanical crisis was certainly bad news for farmers. Regardless, the scientist was strangely awed at the plant’s evolutionary capabilities. He actually admitted that he was excited to see it, referring to the moment his team discovered a new case of mutated herbicide tolerance.
Genetic engineers first created glyphosate-tolerant soybeans in 1996. Farmers enthusiastically welcomed the paired products – herbicides and crops tolerant of those chemicals. By 2011, the USDA’s Economic Research Service stated that about 95% of all soybean acres in the U.S. were planted with seeds engineered to resist herbicides. Cotton and corn followed similar trajectories.
Between 1990 and 2014, the volume of U.S. glyphosate use increased by a factor at least 30. “It was so cheap and effective that it was all that people used for almost two decades,” according to Stephen Duke, a former USDA researcher. Turns out that A. palmeri – a member of the pigweed family – was perfectly adapted to evolve glyphosate resistance, doing so quickly.
This weed is native to the Southwest, and its leaves were once baked and eaten by different Native American tribes, like the Navajo, who ground the seeds into meal (effectively a type of cereal). But as the pigweed variant spread eastward, the plants started competing with cotton in the South, becoming a serious threat to that crop by the mid-1990s.
With modern biotechnology, where cash crops are uniformly identical – i.e., natural biodiversity has vanished – farmers, every year, purchase new genetically engineered seeds containing the glyphosate-tolerant trait.
In this scenario, A. palmeri takes advantage of its own incredible genetic diversity. Its reproductive behavior is classed as dioecious – these plants mate sexually, or via obligate out-crossing, in biologist jargon. Each female plant can produce hundreds of thousands of seeds annually. The plants that sprouted, blessed with random mutations, that equipped them to survive herbicide doses survived to reproduce with each other. Then, once applications of glyphosate obliterated all the weeds in a field – except for the glyphosate-tolerant A. palmeri specimens – the surviving pigweeds could spread without competition.
In one study, researchers planted a single glyphosate-resistant A. palmeri seedling in each of four fields of genetically modified cotton. In three years, the weeds choked out the cotton in all four fields, thus destroying the crop. Eventually, according to Brown, glyphosate was soundly outfoxed by A. palmeri’s evolutionary resilience.
Glyphosate resistant strains of this weed quickly spread through the South, then moved north, stowing away in cottonseed hulls used for animal feed. Once consumed, these tiny seeds passed through digestive systems of the livestock consuming them. Farmers who spread the contaminated cow manure on their fields – an almost universal practice as an economical form of fertilizer – unwittingly assisted A. palmeri’s spread.
This skilled opportunist now grows in at least 39 states. The only states in the Northeast where the weed isn’t a nuisance are Maine, Vermont, New Hampshire, Connecticut and Rhode Island.
It boasts tremendous biodiversity, dioecious reproduction and voluminous seed production. As a member of the C-4 grouping, its structural carbon configuration ensures that it’s an extremely efficient moisture conserver.
I believe that if plant population genetics is an academic subject, many of its otherwise learned scientist students need to enroll in summer school.